Low light level video camera systems utilizing image intensifiers have long been used for night surveillance or investigation. Heretofore, such known systems have been rather bulky. Also, such low light level TV pick-up systems are adapted for operation only under low light level conditions and due to the image intensifier, cannot operate under lighting conditions where the ambient light is above a given level. Each basic system has merit in a limited area but none to date are known to use to best advantage the multiple sciences available.
Low light level video camera systems comprise a lens followed by a light intensifier and a vidicon. All electrostatic intensifiers use fibre optic plates on their input and output. This requires the use of a fibre optics faceplate on the intimately coupled vidicon or a fibre optic coupling member as disclosed in U.S. Pat. No. 3,553,363. Thus, the image scanned by the vidicon is the fibre optics plate of the intensifier or the fibre optic coupling member. In these arrangements, the gain of the intensifier is controlled by controlling the high voltage on the intensifier as a function of a vidicon signal. Such an arrangement is disclosed in U.S. Pat. No. 4,025,955.
One system makes use of a multiple of electrostatically focused intensifiers in tandem whose output is intimately coupled to a pickup tube (generally a vidicon). The output from the vidicon is used to provide the conversion to a video signal for modulating a cathode ray tube after amplification to the required levels. This system (one-two or three electrostatic image intensifiers in tandem) provides light gains in the order of 200 to 8 million. These gains are unrealistic and unusable. Noise from the first intensifier is amplified by the second and then again amplified by the third. The end result is that the signal to noise ratio is primarily defined by the first intensifier and therefore the gain of each state must be limited. Effective gains in the order of 40-100,000 are realistic but above gains of 25,000 amplified noise inhibits the low light usability.
A second system makes use of what is generally called a second generation intensifier. The second generation differs from the electrostatically focused intensifier by the addition of a microchannel plate amplifier between the photocathode input and phosphor screen output. Gains in the order of 35-70,000 are effectively realized with equivalent noise inputs of 1.times.10.sup.-11 foot candles.
All electrostatic intensifiers use fibre optics plates on their input and output. This then requires the use of a fibre optics faceplate or coupler on the intimately coupled vidicons used in low light level video systems. Such arrangements are exemplified in U.S. Pat. Nos. 3,553,363 and 3,974,331.
A third system utilizes what is called a silicon-intensified-target vidicon or an intensified silicon-intensified-target. The gains of these third systems are nearly equal to the second-generation-vidicon arrangement but lack an important effect of the latter.
The second-generation intensifier output is a visible image from a phosphor screen, the decay time of which provides integration of the electron images between electron beam scans of the vidicon target. The third systems are non-integrating, producing a signal that must be stored externally in the form of multiple scans to realize similar signal to noise ratios.
To realize the potential resolution and low light capability of a second-generation intensifier, it requires an accellerating potential such that a bright image is displayed for presentation to the vidicon. The dynamic range of this output image is limited to a level of about five magnitudes.
The vidicon pickup tubes on the other hand are effective over about three to four magnitudes. In a close (intimate) coupled system, the signal from the vidicon is compressed at the higher brightness levels by target current limiting. The result is poor interscene dynamic range and blooming of highlights. Sulfide target vidicons have reasonable dynamic range but have poor noise ratios at low brightness levels. Silicon target vidicons have excellent noise ratios at low levels but have limited dynamic range.
Accordingly, the present invention provides a compact, new and improved low light level video camera system which may also be operated during daytime without damage to the vidicon tube and intensifier.